This invention relates to an exposure-control device for use in a field-sequential color video camera.
In recent years, such field-sequential color video cameras have been applied to medical instruments, particularly to endoscopes. A video camera applied to an endoscope picks up the image of an object by means of a solid-state image sensor, such as a charge-coupled device (CCD) incorporated at the distal end of the endoscope. The output signals from the CCD are transmitted via signal lines in the endoscope to an external monitor or a display device, where they are displayed. This type of endoscope adopts a field-sequential system as a color image pick-up or camera system, since it performs dedicated illumination for observing dark objects such as the interior of body cavities and of fine tube members, and has a CCD incorporated in the narrow area of its distal end.
The construction and operation of this field-sequential system will be briefly explained below.
A rotary filter is provided at the front surface of a light source lamp which provides incident illumination for the light-guide fiber of the endoscope. The rotary filter serves to color the illumination light, sucessively red (R), green (G), and blue (B), with a light-blanking period occurring between each illumination period. The rotary filter makes one rotation during each field period of the standard television system, and as it does so, the illumination light is sequentially colored R, G, and B. The image signals corresponding to the respective color components of R, G, and B are superposed to be synthesized to form the full-color image signal of one field. Namely, the three color component fields picked up by the CCD are displayed as one field.
Generally, the color filter components corresponding to the respective colors of the rotary filter are not arranged successively, but are separated by portions which shade the illumination light. Thus, the image information corresponding to each individual color component is stored in the CCD during the illumination or coloring period associated with that particular color, and thereafter is read out during the subsequent light-shading period. The amount of incident illumination light emitted by the light-source lamp to the light-guide fiber is automatically adjusted, by means of a diaphragm, in accordance with the brightness level of the object under examination, since in the body cavities, the reflection factor, and hence the level of brightness, vary greatly. The brightness level of the object is ascertained by integrating the image signal provided by the CCD over a one-frame pick-up period.
Although the aforementioned field-sequential color video camera has been widely applied to a variety of medical instruments, it possesses the following defect:
In this camera, a one-field full-color signal consists of the color components of R, G, and B output from the CCD during the light-shading periods which occur between each associated illumination period. The image signal from the CCD is at zero-level during the illumination periods. Therefore, when the image signal from the CCD is integrated over a one-frame period, an integration capacitor, charged by the output image signal from the CCD, is discharged toward the CCD during these illumination periods. Thus, the light-adjust signal resulting from the integrated outputs will fluctuate during the image pick-up field period, thereby rendering proper diaphragm control impossible. Incidentally, this applies to not only the aforementioned electronic scope but also to general field-sequential color video cameras using an image pick-up tube. This phenomenon also occurs with a color mosaic filter type color video camera if the brightness of one frame image is determined by integrating two fields of image signals, since the blanking period seems to be the same as the illumination period.